Boris Johnson only candidate who can ensure Tories will win next General Election, polling suggests

Boris Johnson is the only candidate for the Tory leadership who has enough support from ordinary voters to ensure the Conservatives win the next general election, private polling shows.

Mr Johnson, who led the Leave campaign alongside Michael Gove, is the only senior Tory with the wider appeal necessary to ensure an election victory, data has shown.

The former Mayor of London also has the support of crucial Conservative donors who believe he is best-placed to build a strong party machine ahead of any election, sources close to the former mayor of London said.

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The likely replacement of Jeremy Corbyn as Labour leader has led to senior Tories becoming increasingly convinced that the next general election is no longer a foregone conclusion.

It means that the Conservative leader will need to be able to reach out to swing voters across the country, which many party figures believe only Mr Johnson can do.

It is expected that Mr Johnson will on Thursday declare that he will stand in the Tory leadership contest to succeed David Cameron.

Liz Truss, the Environment Secretary, uses an article in The Telegraph to become the first Cabinet minister to declare for a candidate, saying that Mr Johnson and Michael Gove, the Justice Secretary, can "bring the country together".

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Ms Truss, who supported the campaign to keep Britain in the EU, said that the next Conservative leader must be someone from the Leave side of the referendum campaign.

She hailed Mr Johnson's vision of a "prosperous, modern, liberal Britain" outside the European Union and said that Michael Gove, the Justice secretary who is backing Mr Johnson, will "get results" from Europe.

Mr Johnson’s main challenger is likely to be Theresa May, the Home Secretary, who wants to focus her campaign on national security arguing that she is best-placed to guide the country through the challenging times ahead.

A new poll of more than 1,300 readers of the Conservativehome website put Mrs May narrowly ahead of her arch rival Boris Johnson by 29 per cent to 28 per cent.

However, a series of other candidates, including Stephen Crabb, the Work and Pensions Secretary, and Nicky Morgan, the Education Secretary, will also declare their candidacy, the Telegraph understands.

Leave campaigners Andrea Leadsom and Priti Patel, who were said to be considering leadership challenges, are now understood to be preparing to support Mr Johnson, this newspaper understands.

Both Mr Johnson and Mrs May have ruled out a snap general election if they win the leadership contest amid concerns that it could jeopardise the Conservative's majority and distract from referendum negotiations.

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On Tuesday, it emerged that Sir Lynton Crosby is preparing to advise Mr Johnson if he declares his intention to join the leadership contest.

The Australian election strategist, who masterminded the Conservative Party's 2015 general election victory, is understood to have met Mr Johnson in recent days to discuss his plans.

The Telegraph understands that Sir Lynton has agreed to take a role in the campaign if Mr Johnson declares his candidacy later this week.